Granule Formation Mechanisms within an Aerobic Wastewater System for Phosphorus Removal

Abstract

Granular sludge is a novel alternative for the treatment of wastewater and offers numerous operational and economic advantages over conventional floccular-sludge systems. The majority of research on granular sludge has focused on optimization of engineering aspects relating to reactor operation with little emphasis on the fundamental microbiology. In this study, we hypothesize two novel mechanisms for granule formation as observed in three laboratory scale sequencing batch reactors operating for biological phosphorus removal and treating two different types of wastewater. During the initial stages of granulation, two distinct granule types (white and yellow) were distinguished within the mixed microbial population. White granules appeared as compact, smooth, dense aggregates dominated by 97.5% "Candidatus Accumulibacter phosphatis," and yellow granules appeared as loose, rough, irregular aggregates with a mixed microbial population of 12.3% "Candidatus Accumulibacter phosphatis" and 57.9% "Candidatus Competibacter phosphatis," among other bacteria. Microscopy showed white granules as homogeneous microbial aggregates and yellow granules as segregated, microcolony-like aggregates, with phylogenetic analysis suggesting that the granule types are likely not a result of strain-associated differences. The microbial community composition and arrangement suggest different formation mechanisms occur for each granule type. White granules are hypothesized to form by outgrowth from a single microcolony into a granule dominated by one bacterial type, while yellow granules are hypothesized to form via multiple microcolony aggregation into a microcolony-segregated granule with a mixed microbial population. Further understanding and application of these mechanisms and the associated microbial ecology may provide conceptual information benefiting start-up procedures for full-scale granular-sludge reactors.Activated sludge can be operated as an efficient biological process to treat and remove unwanted nutrients, such as carbon, nitrogen, and phosphorus, from wastewater. Two common processes applied in wastewater treatment are enhanced biological phosphorus removal (EBPR) (28), which is widely applied for carbon and phosphorus removal, and simultaneous nitrification-denitrification and phosphorus removal (SNDPR) (40), which combines carbon, phosphorus, and nitrogen removal. Conventional plants operating for EBPR and SNDPR typically operate as floccular-sludge systems, in which small aggregates (30 to 200 m) of microorganisms make up the suspended biofilms of activated sludge (10). Aerobic granular activated sludge is a novel alternative for the treatment of wastewater and offers several operational and economic advantages over conventional floccular-sludge systems (9, 22). Granular sludges are typically larger suspended biofilm aggregates (200 to 2,000 m) and typically contain higher biomass concentrations within the same reactor vessels than floccularsludge systems (4, 29). However, granular sludge has yet to be applied to full-s...

The role of quorum sensing signalling in EPS production and the assembly of a sludge community into aerobic granules

“…Interestingly, although the same operational parameters were used throughout the experiment, the granules dispersed at phase V (Figure 1). Whereas, dispersal may be related to cell lysis induced by predators, for example, phages (Barr et al, 2010), the dispersal phenomenon was clearly linked to a decrease in AHL concentration in this study (Figure 2). The loss of signals could be the consequence of decreased competiveness of cluster 2 organisms, which were closely associated with QS signalling, resulting in a reduction in AHL controlled EPS production.…”

“…Development of a complex, functional granular sludge community Granules are defined as compact aggregates that are characterized with a minimum particle size of 100 mm in diameter and have a SVI 5 (a measure of biomass density/compactness) of 50 ml g À 1 or less (Barr et al, 2010). Samples were collected weekly for image analysis, quantification of SVI 5, particle size and sludge biomass to monitor the granulation process, and five distinct phases of granular development were defined (Figure 1).…”

Metagenomic and metaproteomic analyses of Accumulibacter phosphatis‐enriched floccular and granular biofilm

“…This first reactor, called "Floc" ) for the enrichment of PAO. All other operating conditions are as previously described (Barr, Cook, et al, 2010).…”

“…Catalase HPII was the most highly enriched protein in Gran biofilms and is synthesized in response to metabolic growth on TCA cycle intermediates, rather than in response to peroxide (Loewen et al, 1985). The enrichment of stress and starvation proteins within Gran proteomes suggests that the microenvironment within the granular biofilm is nutrient and substrate limited, leading to controlled cell death in the inner most regions of granules (Barr, Cook, et al, 2010). This is likely a result of mass transfer limitations and substrate availability patterns that occur within the larger biofilm (Liu et al, 2005;Toh et al, 2003).…”

Aerobic Granules: Microbial Landscape and Architecture, Stages, and Practical Implications

“…Although this approach has increased our understanding of the process, models are often only validated against macroscale results with limited or no microscale observations (21,22). Despite the fact that the physical architecture of the granules is intrinsically related to mass transport, conversion processes and niches partitioning within the granules, few reports describe details on the microstructure of aerobic granules (23,24). In addition, an effective approach to stain all of the constituents of the aggregates is required because most previous research has focused on fluorescence methods to target specific bacterial types.…”